A key function in wireless networks is link adaptation (LA), which is also referred to as adaptive modulation and coding (AMC). Link adaptation is aimed at estimating, based on available channel information, the most appropriate modulation order and coding rate to be used at a given time on a radio link to meet a target criterion, such as a block error rate (BLER). Fast link adaptation (FLA) was introduced in CDMA and GPRS systems and was later extended to high-speed downlink packet access (HSDPA) systems. FLA provided better utilization of instantaneous channel capacity since wireless channels are time-varying and frequency-selective. FLA relies on periodic measurements of the signal to interference plus noise ratio (SINR). This is also referred to as the channel quality indicator (CQI) of the access link. FLA essentially consists of comparing every new SINR update with a number of switching thresholds that partition locally-optimum modulation and coding schemes (MCS) along the SINR dimension. This allows for selection of the best MCS for that SINR.
In real systems, instantaneous SINR values include feedback delay (in addition to scheduling delay), estimation errors, and varying channel statistics. As a result, the SINR thresholds need to be adapted at a slower rate) to track and mitigate these imperfections. Algorithms for adapting the SINR thresholds are based on hybrid automatic repeat request (HARM) results (ACK/NACK) from the actual transmissions. The fast MCS selection and complementary slower threshold adaptation of the FLA are often referred to as Inner-Loop and Outer-Loop LA (ILLA/OLLA), respectively.
LA also plays an essential role in the Long Term Evolution (LTE) wireless networks, also known as Evolved Universal Terrestrial Radio Access Network (E-UTRAN), which are standardized by the 3GPP working groups. OFDMA and SC-FDMA (single carrier FDMA) access schemes are used for the down-link (DL) and up-link (UL) of E-UTRAN, respectively. This access scheme allows greater bandwidth efficiency (in contrast to WCDMA) with dynamic frequency aware radio resource allocation.
Allocation decisions are made by an LTE scheduler for every 1 ms transmission time interval (TTI). The allocation decisions are based on the expected instantaneous user throughput at a potential future transmission time for a selected modulation and coding scheme (MCS) as predicted by the LA function. Since LTE adds the frequency and space, for example Multiple Inputs Multiple Outputs (MIMO), dimensions in the scheduling space, research in LA for LTE downlink has mainly focused on the associated feedback schemes. Most basic principles of the HSDPA ILLA and OLLA were adapted to EUTRA DL, such as applying and adapting a backoff to the User Equipment's (UE) SINR measurements rather than adapting the MCS SINR switching thresholds.
In contrast, little focus was given to evolved universal terrestrial radio access (EUTRA) UL, which differs from the DL in several aspects pertinent to LA, such as: single-carrier access scheme, UL SINR is explicitly computed by the Base Station (or eNodeB) from various UL reference signals, synchronous HARQ is used with the possibility to send non-adaptive retransmissions, the UL interference is more dynamic, UE can be power limited, etc.